This invention relates to a multichip presscontact type semiconductor device used in a state where a plurality of semiconductor chips press-contacted each other, in particular, a device suitable as a multichip press-contact type IGBT (Insulated Gate Bipolar Transistor) for controlling the motor drive of an electric train or traction inverter application for which high withstand voltage and high reliability are required.
The conventional multichip press-contact type IGBT has a construction such as shown in FIG. 1. FIG. 1 is a sectional view of the enlarged main portion of the device taken out from a package. Such a press-contact type semiconductor device is described, for example, in U.S. patent Ser. Nos. 08/656,868, 08/665,980, and 08/526,320, which are disclosed by the inventors of the present invention, and so on.
A FRD (First Recovery Diode) chip 12 is connected in parallel to a IGBT (Insulated Gate Bipolar Transistor) chip 11 having the current flowing direction opposite to that of the FRD chip 12. The plain where the chip 11 and chip 12 are disposed is provided thereon with a plurality of the chips 11 and 12 although FIG. 1 does not show this. A chip frame 13 fixes and positions the IGBT chip 11 and the FRD chip 12 and fixes them. Heat buffer plates 14 each made from materials such as Mo (molybdenum) are located on the main surfaces of the chips 11 and 12 so as to radiate heat from the main surfaces of the chips. Similarly, a heat buffer disk plate 15 made from materials such as Mo is provided on the rear surfaces of the chips 11 and 12 so as to radiate the heat from the rear surfaces of the chips. A resin frame 16 having an opening facing the IGBT chip 11 and the FRD chip 12 and the heat buffer disk plate 15 hold the chips 11 and 12 therebetween such that the chips 11 and 12 are positioned and fixed in a vertical direction. A ring frame 17 has an opening at a position corresponding to a collector press-contact electrode plate 28. The opening of the ring frame 17 is engaged with the resin frame 16, thereby the chips 11 and 12 and the heat buffer disk plate 15 are held by the ring frame 17 and the resin frame 16. On the rear surface of an emitter press-contact electrode plate 27 which faces the chips 11 and 12, a separation trench for making the pressure applied to each of the chips even is formed at a non-press-contact portion where the emitter press-contact electrode plate 27 are not connected to the chips. In the trench, the column protrusions corresponding to the chips 11 and 12 are formed. The emitter press-contact electrode plate 27 and the collector press-contact electrode plate 28 are applied with a pressure as highas 10 MPa to use the chips 11 and 12, heat buffer plate 14, and heat buffer disk plate 15 in the press-contact state. A contact probe 30 contained in a sheath-like insulator 31 is arranged to contact the gate electrode of the IGBT chip 11. The contact probe 30 is connected to the gate wiring to which a control signal is supplied.
FIG. 2 presents an equivalent circuit diagram of the multichip press-contact IGBT described above. The IGBT chip 11 having a MOS gate structure has capacitance Cge between the gate and the emitter, capacitance Ccg between the collector and the gate, and capacitance Cce between the collector and the emitter. When the plurality of IGBT chips 11 are connected in parallel, a parasitic inductance Le is generated in the emitter wiring connecting the chips 11, and a parasitic inductance Lc is generated in the collector wiring connecting the chips 11. Similarly, when a plurality of FRD chips 12 are connected in parallel, a parasitic inductance La is generated in the anode wiring connecting the chips 12, and a parasitic inductance Lk is generated in the cathode wiring connecting the chips 12.
As is clear from the above-mentioned constitution, when a plurality of IGBT chips and a plurality of FRD chips are connected in parallel, the LCR oscillation circuit is constituted by the capacitances, parasitic inductances, and parasitic resistances in the circuit. The LCR oscillation circuit needs to be prevented from oscillating in operating the device.
In general, with the constitution shown in FIG. 2, the LCR circuit can easily oscillate unless the parasitic inductance Lc in the collector wiring is not set larger than the parasitic inductance Le. However, in the multichip press-contact type IGBT constituted as shown in FIG. 1, the parasitic inductance Le is necessarily set larger than the parasitic inductance Lc in the collector wiring due to the structural limitation. The emitter wiring (the current path in the emitter press-contact electrode plate 27) connecting the chips is lengthened due to the separation trench which is formed on the rear surface of the emitter press-contact electrode plate facing the chips in order to make the pressure applied to each of the chips even. When the LCR circuit constituted by the parasitic inductances, capacitances, and parasitic resistances is triggered to oscillate by noise and the like, errors may occur in the operation of the IGBT chips 11, and the IGBT may be damaged.
In order to form the above-mentioned multichip press-contact type IGBT chips with a high withstand voltage, the collector and the emitter need to be separated by a predetermined creepage distance. Generally, the creepage distance is determined as 1 mm/1 kV. Therefore, for the withstand voltage of 1 kV, the creepage distance as long as 1 mm is needed. In the device with the withstand voltage as high as 2500V, the creepage distance of at least 2.5 mm needs to be provided between the collector and the emitter. When the collector and the emitter in the structure shown in FIG. 1 are formed with a sufficiently long creepage distance therebetween, the heat buffer plate 14 needs to be formed thick, and thus the thermal resistance increases in the buffer plate. The increase of the thickness of the heat buffer plates 14 lengthens the emitter wiring, and thus the parasitic inductance Le in the emitter wiring increases as the withstand voltage of the device increases. Therefore, in the conventional device, if the heat buffer plate is formed thick to obtain the sufficient creepage distance, the parasitically constituted LCR oscillation circuit may be easily oscillated.